Surgical alignment by magnetic field gradient localization

ABSTRACT

A three dimensional magnetic sensor attached to a surgical nail is located based on an applied monotonic magnetic field gradient. Another three dimensional magnetic sensor locates a surgical drill. A display generates a real time image of the relative alignment of the surgical drill and of the surgical nail, allowing a surgeon to repair bone fractures.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 62/688,235, filed on Jun. 21, 2018, the disclosure ofwhich is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to magnetic sensors. More particularly,it relates to surgical alignment by magnetic field gradientlocalization.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of this specification, illustrate one or more embodiments of thepresent disclosure and, together with the description of exampleembodiments, serve to explain the principles and implementations of thedisclosure.

FIG. 1 illustrates a nail in a bone.

FIG. 2 illustrates typical dimensions of an ATOMS device.

FIG. 3 illustrates a schematic of ATOMS positioning.

FIG. 4 illustrates an exemplary field gradient for positioning.

FIG. 5 illustrates two power delivery arrangements.

FIGS. 6-8 illustrate a set up for the magnetic gradient coils.

FIGS. 9-11 illustrate magnetic field gradients.

FIG. 12 illustrates time multiplexing.

FIG. 13 illustrates an exemplary PCB with a Hall sensor.

FIG. 14 illustrates a simulation setup.

FIGS. 15-16 illustrate an exemplary ASIC implementation.

FIG. 17 shows the simulation results for the PMU unit.

FIG. 18 shows the RF-wake-up block waveforms.

FIG. 19 shows the waveforms of the data acquisition unit.

FIG. 20 shows the backscatter modulation waveforms.

FIGS. 21-22 illustrate exemplary coils.

FIGS. 23-25 illustrate simulated field gradient plots.

FIGS. 26 and 27 illustrate a comparison between simulated and measuredfield values for exemplary coils.

FIG. 28 shows a system overview.

SUMMARY

In a first aspect of the disclosure, a system is described, the systemcomprising: a first sensor configured to be inserted in a patient duringa surgical procedure, the first sensor comprising: a first magneticsensor configured to detect a first magnetic field value, a firstintegrated circuit chip configured to process data from the firstmagnetic sensor, and a first radiofrequency coil configured to transmitdata processed by the first integrated circuit chip based on the firstmagnetic field; a second sensor attached to a surgical instrument, thesecond sensor comprising: a second magnetic sensor configured to detecta second magnetic field value, and a second integrated circuit chipconfigured to process data from the second magnetic sensor; and aplurality of coils configured to generate a magnetic field gradientwithin a volume in which the surgical procedure takes place, wherein themagnetic field gradient has a unique field value at each spatiallocation.

In a second aspect of the disclosure, a system is described, the systemcomprising: a first sensor configured to be inserted in a patient duringa surgical procedure, the first sensor comprising: a magnetic sensorconfigured to detect a first magnetic field value, an integrated circuitchip configured to process data from the magnetic sensor, and aradiofrequency coil configured to transmit data processed by theintegrated circuit chip based on the first magnetic field; a surgicalinstrument; a second sensor configured to sense a location of thesurgical instrument relative to the first sensor; and a plurality ofcoils configured to generate a magnetic field gradient within a volumein which the surgical procedure takes place, wherein the magnetic fieldgradient has a unique field value at each spatial location.

In a third aspect of the disclosure, a method is described, the methodcomprising: providing a system comprising: a first sensor configured tobe inserted in a patient during a surgical procedure, the first sensorcomprising: a first magnetic sensor configured to detect a firstmagnetic field value, a first integrated circuit chip configured toprocess data from the first magnetic sensor, and a first radiofrequencycoil configured to transmit data processed by the first integratedcircuit chip based on the first magnetic field; a second sensor attachedto a surgical instrument, the second sensor comprising: a secondmagnetic sensor configured to detect a second magnetic field value, anda second integrated circuit chip configured to process data from thesecond magnetic sensor; generating, by a plurality of coils, a magneticfield gradient within a volume in which the surgical procedure takesplace, wherein the magnetic field gradient has a unique field value ateach spatial location; sensing, by the first sensor, a first location ofthe first sensor based on the magnetic field gradient; sensing, by thesecond sensor, a second location of the second sensor based on themagnetic field gradient; displaying, on a display, the first and secondlocations, and a relative alignment between the first location and thesecond location; and aligning the surgical instrument based on thedisplayed relative alignment.

In a fourth aspect of the disclosure, a method is described, the methodcomprising providing a system comprising: a first sensor configured tobe inserted in a patient during a surgical procedure, the first sensorcomprising: a magnetic sensor configured to detect a first magneticfield value, an integrated circuit chip configured to process data fromthe magnetic sensor, and a radiofrequency coil configured to transmitdata processed by the integrated circuit chip based on the firstmagnetic field; a surgical instrument; and a second sensor configured tosense a location of the surgical instrument relative to the firstsensor; generating, by a plurality of coils, a magnetic field gradientwithin a volume in which the surgical procedure takes place, wherein themagnetic field gradient has a unique field value at each spatiallocation; sensing, by the first sensor, a first location of the firstsensor based on the magnetic field gradient; sensing, by the secondsensor, a location of the surgical instrument relative to the firstsensor; displaying, on a display, the first and second locations, and arelative alignment between the first location and the second location;and aligning the surgical instrument based on the displayed relativealignment.

DETAILED DESCRIPTION

The present disclosure refers to the surgical alignment of fracturedbones. Intramedullary (IM) nailing is the process of treating long bonefractures. It consists of insertion of a metallic nail into themedullary canal of the fractured bone, followed by locking screws toavoid displacement of bone fragments around or along the nail. Thesurgical nail comprises holes to accept the screws. The screws areinserted through bone fragments into the proximal and distal holes inthe nail. Proximal screw locking is performed using a mechanical guidefixed to the proximal part of the nail, and is relatively simpler thandistal locking. Using such a guide is not possible for distal lockingbecause the nail is usually deformed during its insertion into thecanal, owing to the non-linearity of the bone. This deformation can beas high as 10 mm from the axis of the nail. Therefore, distal locking isthe most challenging part of intramedullary nailing of femur and tibia.Additional pitfalls and complications that may occur include inadequatefixation, bone cracking, cortical wall penetration, bone weakening dueto multiple or enlarged screw holes, and distal fragment mal-rotation ifthe drill is not perfectly aligned with the hole axis.

In order to locate the distal holes, various methods have been proposedand are used by surgeons worldwide. The most frequently used is thefreehand technique in which the hole axis is determined using a surgicaldrill and the alignment is achieved through fluoroscopic imaging. Thefluoroscopic device is positioned perpendicular to each distal hole sothat it appears perfectly circular on the screen. This alignmentprocedure is time-consuming and exposes the patient and the surgicalteam to high irradiation. The surgeon's direct radiation exposure variesfrom 3.1 min to 31.4 min, with the distal locking itself causing 31%-51%of the total irradiation per nailing operation. Moreover, the free handtechnique requires the expertise of both the surgeon and the X-raytechnician; it has a slow learning curve, and it is largely dependent onthe image intensifier.

Various other methods which minimize or completely eliminate irradiationduring distal locking have been proposed. These include hand heldtargeting devices and radiolucent drill guides, laser-guided systems,computer-assisted systems, image intensifier mounted targeting devices,proximally mounted distal locking devices, and electromagnetic fieldtracking technology. These proximally mounted targeting devices failbecause their aiming arms do not compensate for a significantdeformation of the nail caused during insertion. Other methods are evenmore complex and their successful use requires a significant learningcurve for the surgeon and the staff. Moreover, additional requirementslike computing system, robotic arm, computed tomography (CT) images,sophisticated hardware and software, make these propositions expensiveto be implemented widely. For these reasons, the most familiar methodremains the freehand technique, despite all its limitations. However,there is a clear need for alternative techniques with limited or noradiation exposure in distal locking of IM nails.

The present disclosure describes the design of a fully implantablewireless electronic device which can be used to eliminate fluoroscopicimaging in the IM nailing process, and also reduce the present surgerytime since continuous monitoring to get accurate images during distallocking would no longer be required.

The present disclosure describes a wireless electronic device which canprovide accurate 3D position information when a magnetic field gradientis applied across it. This device is referred to as AddressableTransmitters Operated as Magnetic Spins, abbreviated as ATOMS. Thedevice can be placed on an IM nail right next to the distal hole, asshown in FIG. 1. FIG. 1 illustrates the ATOMS device (105) attached tothe nail (110), and two screws (115) inserted within holes in the nail.

A 3D magnetic field gradient is applied over the distal nail position.The 3D magnetic sensor, integrated in ATOMS, senses the magnetic fieldat its location and transmits the corresponding value. This value, whenmapped to the externally applied gradient, gives the exact position ofthe sensor. The schematic of the working principle is shown in FIG. 4.FIG. 4 illustrates an exemplary magnetic field gradient (405), having avarying value across the bone, which is sensed by the ATOMS (410). Forexample, coils can be placed under a patient's leg on an operatingtable, generating a magnetic field gradient within the volume of theleg.

FIG. 2 shows typical dimensions of an ATOMS device, compared to a hole(210) in a nail. The present disclosure describes how to provideaccurate 3D navigation data from ATOMS, enabling the surgeon to obtaincomplete information about its position in 3D space, which is equivalentto having the distal hole's position. The drill bit which is used tocreate the initial opening in the bone to insert a locking screw isaccurately navigated to the correct position of the targeted hole insuch a way that the axis of the hole matches the axis of the drill bit.To ensure perfect alignment, another identical ATOMS is installed in thedrill bit along the drilling axis. The goal is then to bring the twocompletely in alignment with each other, by displaying on a computerscreen the real time position of both of the ATOMS devices.

FIG. 3 shows an overview with two ATOMS (305,310), a drill and thecomputer screen to be used during the operation; ATOMS 1 (305) islocated inside the human body, and ATOMS 2 (310) is on the drill. The 3Dlocation of both is displayed on the computer screen (315). FIG. 28illustrates a system overview comprising a nail (2805), a magneticgradient bed (2815), a drill bit (2810), and a computer display (2820).

An ATOMS sensor can comprise the following components: a 3D magneticHall sensor; an integrated circuit (IC) chip; a power source; and aradiofrequency (RC) coil. The 3D magnetic Hall sensor is used to sensethe magnetic field at the location of the ATOMS. Positioning works byhaving unique field values at all the points of interest, to allow aone-to-one mapping between positions and magnetic field values. Thesensor output is a digital field value which is given to the IC chip forfurther processing. In order to have good spatial resolution, a highresolution 3D magnetic hall sensor is used, so that the requiredmagnetic field gradients remain within an acceptable range.

The IC chip includes the circuitry for transmission of data valuesthrough a chosen standard for wireless communication. A challenge hereis to ensure that communication is done in a power efficient way.Therefore, data telemetry can be carried out through backscattering, atechnology for wireless data communication in low power applications.The chip also carries out timing generation, allowing data measurementsin a time multiplexed fashion, to not incur unnecessary power losses.Time multiplexing requires wake up and sleep signals for the variouscomponents of ATOMS, which are generated by the IC chip.

The power source can employ wireless power transfer as well asbattery-based solutions, as illustrated in FIG. 5. In the example ofFIG. 5, a RF coil (515) is present around a perimeter of the ATOMS, withthe battery, magnetic sensor, and IC chip in the internal area to reducethe overall volume requirements. Wireless power transfer for abiological tissue depth of 10 cm (a typical IM nail location) entailslarge tissue absorption. Therefore, the battery based power supply canbe considered as an alternative solution.

An RF coil can be used for wireless communication throughbackscattering. The RF coil on ATOMS can sense an RF signal and modulateit accordingly, to convey the field value data to an external receiver.The RF coil is also needed when using wireless power transfer to ATOMS.In some embodiments, the wireless power transfer (WPT) is turned offduring data transmission.

In some embodiments, the 3D magnetic sensor has a high magnetic fieldresolution, for example of 1.1 μT or better; a low power consumption,e.g. an average of 10 μW or less; a high field measurement dynamic rangein the three axes components of the fields Bx, By and Bz (e.g. ±35 mT);a 16-bit data resolution for the in-built ADC for each measurementdirection; a low leakage current (e.g. 2 nA); and selectable sensormeasurement range and sensitivity setting. In some embodiments, thesensor may have a I²C bus interface with 4-wire SPI. For example, acommercially available sensor with the above characteristics in a 16-pinQFN package has dimensions of 3.0 mm×3. mm×0.75 mm. On-chip 3D magneticsensors can also be used for this application as this arrangement canmake the system more compact. Important characteristics are the sensorpower, which needs to be low, and the sensor resolution, which should behigh enough to allow working with relatively low magnetic fieldgradients.

The fundamental relationship between the spatial resolution (Δx) of thesensor and the corresponding magnetic field gradient (G_(x)) required toobtain this resolution, given the minimum field value the sensor canmeasure accurately (ΔB_(min)), is given by Eq. (1):Δx=(ΔB _(min))/G _(x)  (1)

In Eq. (1), ΔB_(min) is dictated by the specifications of the magneticsensor and the resolution of the ADC used. In some embodiments, for thesensor having the above characteristics, it ranges from 1.1 μT to 3.1 μTor better. In order to obtain a Δx better than 100 μm, the sensorrequires a magnetic field gradient of 30 mT/m for the lowest sensorresolution. In order to obtain the required field gradients, a specificsetup of gradient coils can be used, as shown in FIGS. 6-8. FIG. 6illustrates a top view of the coils, while FIG. 7 illustrates a bottomview, and FIG. 8 a side view. FIG. 6 illustrates x axis coils (605); yaxis coils (610); z axis coils (615). FIG. 7 illustrates x axis coils(705); y axis coils (710); z axis coils (715). FIG. 8 illustrates x axiscoils (805); y axis coils (810); z axis coils (815). As visible in FIG.8, the y coils are in a plane parallel to that of the x coils, and areoriented rotated by 90° relative to the x coils.

With reference to FIGS. 6-8, the x axis coils carry current in oppositedirections with respect to each other, thus creating a magnetic fieldgradient along the x direction for the z axis magnetic field component.Similarly, the two y axis coils lying underneath the x axis coils carryopposite currents with respect to each other, which gives rise to agradient along y direction for the z axis magnetic field component. Thegradient along the z direction for the z axis magnetic field componentis created by the set of z axis coils, all carrying current in the samedirection. The magnitude of the axial magnetic field for the z axiscoils decays along the z axis, thus creating a gradient along the zdirection. The generated field gradients are shown in FIGS. 9-11. FIG. 9illustrates a magnetic field gradient of 30 mT/m in the x direction.FIG. 10 illustrates a magnetic field gradient in the y direction. FIG.11 illustrates a magnetic field gradient in the z direction. The currentdensities used to generate these gradient fields are: 0.54 A/mm² for thex coils, 0.62 A/mm² for the y coils, and 1.1 A/mm² for the z coils.

As observed in FIGS. 9-10, the gradients in the x and y directionsattain their maximum values at x=0 and y=0, respectively. FIG. 11 showsthat the maximum gradient in the z direction occurs at 10 cm, which alsolies at the center of the operational range. This design allows thehighest gradient regions to occur at the center of the operationalrange, corresponding to the location of the hole on the IM nail insidethe bone. The monotonic nature of the magnetic fields ensures a uniquemapping of all points in space to the corresponding magnetic fieldvalues.

The gradient fields have a range large enough to accommodate both of theATOMS devices: the device on the IM nail and the device on the drill.The overlap is especially useful in the initial phase of the surgicalprocedure, when the ATOMS on the drill is far from the intended finallocation. As seen in FIGS. 9-11, the designed coil setup allows aworking range of 15 cm-20 cm, which is more than sufficient for atypical IM nailing surgery. As the drill comes closer to the desiredfinal location, the resolution improves. The coils generating themagnetic field can be placed, for example, under the patient's leg in anoperating theater bed, if the broken bone is within the leg.

Since power delivery to ATOMS can be challenging, especially ifperformed wirelessly, it is useful to reduce power consumption.Therefore, judicious use of power is useful at every step, a major partof which can be achieved by turning on the individual components ofATOMS only when needed, and sending them to a dormant or sleep mode whennot in use. Therefore, a time multiplexed approach for the completesystem is useful in order to reduce the total power required by ATOMS.

The sequence of time-multiplexed measurements of the magnetic fieldgradients in each direction is shown in FIG. 12. Only the x coils areswitched on during the x phase, and similarly for the y and z phases.First, a wake up command is sent to bring the system out of sleep mode.Then, the x gradient coils are turned on and allowed to reach a steadystate value in about 2 ms (ramp-up time due to current switching incoils). Following this step, the field measurement takes place in thenext 1 ms, and then the x gradient coils are turned off. The switchingoff phase takes another 2 ms, thus making the whole step 5 ms long. Thesame set of events is repeated for the y and z gradient measurements.

The 1 ms time window required to measure the field gradient in any givendirection is dictated by the sensor used for magnetic fieldmeasurements. The reported time needed to make an accurate fieldmeasurement by the sensor described above is about 1 ms, thus requiringthe gradients to be stable for at least that much time. This alsoimplies that the sensor has to be turned on only for the desired 1 mstime window for the x phase, and then subsequently for the y and the zphases. Therefore, the total on time for the sensor is 3 ms in a 100 mstime frame. After all three measurements corresponding to x, y and z aremade, the sensor and the coils are turned off for the next 85 ms througha sleep command. The person of ordinary skill in the art will understandthat the above parameter values are exemplary, and if using a differentsensor having different characteristics, a different measurement timewould be employed.

Following the measurements, ATOMS performs data processing and fieldvalue transmission through an RF signal. This step provides a visualfeedback to the surgeon monitoring the updated computer screen (imagerefresh phase) and gives the surgeon enough time for maneuvering thedrill to the updated location. To position the implanted ATOMS withrespect to a reference, a second ATOMS is located on the drill, theposition of which is also constantly displayed on the computer screen.

The complete process makes the duty cycle for the gradient switching tobe 5% for each of the coils, including the switching time. In someembodiments, 10 such measurements can be carried out per second. Thisreduces the time to locate the hole in IM nailing surgery to only acouple of seconds, which is a major advantage over the existingsolutions which take a couple of minutes along with continuousfluoroscopy to locate the hole in the IM nail.

In some embodiments, back scattering is used for wirelessly sending themeasured field data to the external device for position tracking. Asknown to the person of ordinary skill in the art, this form of datatransmission is particularly useful for low power applications. The twomain components of a backscatter communication system are thebackscatter transmitter and a reader. The reader comprises a radiofrequency source along with a backscatter receiver. The RF signal isgenerated by the reader, while the backscatter transmitter modulates andreflects the signal to transmit its data to the backscatter receiver.The modulation produces a digital signal with a varying pulse width,based on the change in effective impedance of the RF coil in response tothe impinging RF pulse. In some embodiments, the RF source for ATOMS canoperate at 13.56 MHz, which lies in the ISM band for biomedicalimplants. The tissue absorption at this frequency is much lower comparedto the higher frequencies normally used for such applications. The Qfactor of the RF coil, which determines the efficiency of the device, isalso high at this frequency.

For the ATOMS device attached to the drill, wireless power transfer anddata-backscattering can be completely eliminated, in some embodiments,by having wired connectivity to the device. Since the drill bit isanyway tethered to a cable, it can be relatively easier to have wiredconnections for power transfer and data communication to the ATOMSdevice instead of performing the same over wireless medium. Anotherpossibility is to perform optical tracking of the drill instead ofhaving an ATOMS device for its positioning. The use of infrared basedlight emitting diodes is known to the person of ordinary skill in theart, for optical tracking of surgical tools. Gyroscope assisted trackingand passive markers like retro-reflective spheres, disks are alsopotential candidates for the same.

As ATOMS can be implanted inside human body, it is essential tohermetically seal the device using a bio-compatible plastic. In someembodiments, encapsulation is carried out with polyetheretherketone(PEEK). PEEK has widespread use in prostheses, dental products andreplacements for metal implants inside human body.

For the timing sequence of FIG. 12, an on-chip clock synchronizes thetime for the sensor and the gradient coils. This synchronization can beachieved by implementing an ultra low power relaxation oscillator whichconsumes a total power of less than 1 nW, an extremely negligible amountcompared to the 10 μW of power consumed by the 3D magnetic sensor. Themagnetic field data obtained from the sensor is used to modulate the RFsignal coming from the backscatter reader. The modulated signal is readby the backscatter receiver located externally. The modulation processis not power hungry and therefore the major power consumption of ATOMScomes from the 3D magnetic sensor.

By adopting a time multiplexed strategy for the field measurement in thex, y and z axes, the average power consumed by the magnetic sensor is0.3 μW as it remains on only for 3 ms out of 100 ms. The other circuitblocks, as described above in the present disclosure, consume negligiblepower compared to the sensor. Therefore, the average power consumptionof ATOMS can be approximately less than 1 μW. For such a low powerrequirement, battery based solutions can be used, as they can easilyprovide the required power for the duration of an IM nailing operation.For example, a rechargeable solid state bare die can be used. It has acapacity of 5 μAh which is more than sufficient for the implanted ATOMSfor a couple of hours, the typical duration of an IM nailing operation.The battery footprint, for example, can be 1.7 mm×2.25 mm×0.2 mm, whichis well within the dimensions of ATOMS and can be easily incorporatedinto the complete system.

For the various current densities used to create magnetic fieldgradients in all the three dimensions, the average heat loss from thecoils is expected to be 8 W. For the given volume of the coils, thiscauses an increase in the coil temperature of less than 0.1° C.,assuming copper metal is used for the coils. This negligible amount ofheat relaxes the constraints on cooling mechanism, which reduces theoverall complexity and cost of the system. The heating issue can beresolved by placing a cooling fan below the coil setup, andencapsulating the complete setup with a thermal insulator.

FIG. 13 illustrates an exemplary PCB with a Hall sensor (1305), and anapplication specific integrated circuit (ASIC) chip (1310), comprising apower management block, and communication modules with the Hall sensorand wake up signals. The 3D magnetic Hall sensor, ASIC chip and the RFcoils are all mounted on a printed circuit board (PCB) which is 10 mm by5 mm (a typical implant size for ATOMS device). The sensor is solderedon the PCB and then connected to the ASIC through copper traces. TheASIC itself is wire-bonded to the PCB using very fine strands of goldwire. The RF coils are mounted all around the PCB and then connected tothe ASIC through copper traces. The back of the PCB contains storagecapacitors which store power received from the coils. These areessential because the most power hungry phenomena during the ATOMSdevice operation is magnetic field measurement by the sensor. Thisoccurs for a very short time frame and the instantaneous RF power isinsufficient to carry out a field measurement successfully. Hence, thestorage capacitors store the power received from the RF coils which canthen be used during the measurement phase. Enough time is providedbetween consecutive measurements for the storage capacitors to re-chargeto their required values. Table 1 lists some exemplary specificationsfor individual components of a sensor.

TABLE 1 Localization resolution 100 μm, 3D Magnetic sensor resolution1.1 μT/LSB (16 bit output) Magnetic sensor dimensions 3 mm × 3 mm × 0.75mm Battery capacity 5 μAh Battery dimensions 1.7 mm × 2.25 mm × 0.2 mmATOMS dimensions 10 mm × 5 mm ATOMS avg. power consumption 1 μWEncapsulating bioplastic PEEK Image refresh rate 10 times per second RFfrequency for data 13.56 MHz transmission Supported range above coil bed15 cm-20 cm Total navigation time 2 hrs

It is possible to model the RF coils to calculate the inductances of theprimary and secondary side coils in the presence of human tissue. Thehuman tissue can be modelled as a composition of muscle, fat and skinhaving a depth of 4 cm, 2.5 cm and 0.5 cm respectively. The primary coilin this simulation is kept 3 cm above the skin, giving a totalseparation of 10 cm between the coils. The complete simulation setup isshown in FIG. 14 and the extracted parameters are listed in Table 2.

As seen from Table 2, the coupling coefficient between the coils inpresence of human tissue is 0.001624. This value can provide a fewmilliWatts of power at the implant depth. In this example, the powerconsumption of the sensor can be restricted to be 1 mW for the completeATOMS device, including the magnetic field sensor.

In some embodiments, the ASIC chip discussed above is responsible for(a) power reception and distribution; (b) data collection, processingand emission; and (c) wake-up when triggered by externally transmittedRF signal. To carry out these three tasks, the ASIC chip has three majorcomponents as described in the following, and as illustrated in FIG. 15:a power management unit, an RF wake up module, and a data acquisitionunit.

The power management unit (PMU) handles the power requirements. The RFpower, for example sent wirelessly at 13.56 MHz, is converted into DCpower using a rectifier, which forms the front-end of the PMU block.Since the power received depends on the distance and alignment betweenthe primary and secondary coils, it is expected to vary during the IMnailing operation. This power variation can lead to much higher voltagebeing induced at the secondary coil when the separation decreases, whichin turn can damage the whole circuitry. In order to keep this induced insafe range, voltage limiters are used in the PMU.

The rectified voltage (typically 0.6 V) may need to be boosted upbecause the subsequent circuit blocks and the sensor usually need highervoltages to operate correctly. The voltage boost can be achieved by acharge pump circuit, which can, for example, boost the rectified voltageby a factor of 4. In some embodiments, the charge pump output voltage isthen fed to regulators to produce three stable and regulated voltages(1520): (a) VDD_S is 2.4 V and is used as supply voltage for the sensor;(b) VDD_A is 1 V and is used as supply voltage for the analog circuitblocks of the ASIC; (c) VDD_D is 0.5 V and is used as supply voltage forthe digital circuit blocks of the ASIC. To generate these supplyvoltages, other circuit blocks like bias voltage generators and currentgenerators can be used. Table 2 lists coil parameters.

TABLE 2 Parameter Primary Coil Secondary Coil Inductance 43.457 μH11.797 μH Quality Factor 272K 97.9 Resistance 13.6 mΩ 10.266Ω CrossSection 5 mm × 5 mm 0.1 mm × 0.1 mm Area 24 cm diamater 4.6 mm × 12 mm Coupling 0.001624 Coefficient

As mentioned above, the RF coils can serve two functions: WPT and datacommunication. The backscatter modulation switch, which connects therectified voltage to charge pump, controls which of the two functionstake place. In the normally ON mode, the switch allows the power to betransferred to the charge pump, and hence facilitates WPT. In the OFFmode, it disconnects the charge pump and modulates the RF signal on thecoil, to backscatter the data to the external receiver.

For the wake-up signal, in some embodiments it is possible to transmitto the ASIC chip a 13.56 MHz RF signal, amplitude modulated with a 17kHz pulse. To decode the wake-up signal from the RF carrier, an envelopedetector first removes the 13.56 MHz frequency component from thesignal, obtaining a 17 kHz pulse at its output. This pulse is furtherprocessed and demodulated in subsequent stages. Since reset signals arealso transmitted to the ASIC in a similar fashion to the wake up pulse,a digital block is used to distinguish the wake up pulse from the resetsignal. The output of the digital block is then used as a wake-up signalto initiate the magnetic field measurement by the sensor, or as a resetsignal which resets the sensor interface block and the sensor.

The sensor is controlled by the data acquisition unit, which sends thecommands needed for its operation. The sensor connects directly to theI2C interface on the ASIC, which follows the standard I2C protocol fordata transfer, known to the person of ordinary skill in the art. Thisdigital module provides the data and clock signals to the sensor andcontrols both of them as per the protocol. The 100 kHz I2C clock isgenerated on the ASIC using a ring-oscillator. The data line isbi-directional and is used for sending and receiving signals from thesensor. The sensor interface module in this unit controls the I2Cinterface module by activating it only when the wake-up signal isreceived. It also collects the data from the I2C interface module aftera measurement is completed. This data is then used to control thebackscatter switch, which modulates the RF signal across the coils intransmission phase.

In some embodiments, a complete ASIC chip can be designed, with a 65nanometer CMOS mixed signal, low power RF process. The complete layoutof the ASIC chip, in this example, measures 1.5 mm by 1 mm and is shownin FIG. 16. FIG. 16 illustrates limiters (1605,1615); a rectifier(1610); a charge pump (1625); a current source (1645); an oscillator(1640); band gap reference (1620); voltage regulators (1630); RF wake up(1635); a regulator (1650); a digital processing block (1655); a I2Cinterface (1660); and a reference voltage generator (1665). The spacesurrounding the above components can also house noise cancellation anddecoupling capacitors.

FIG. 17 shows the simulation results for the PMU unit. The topmostwaveforms (1705) are for the RF to DC power conversion by the rectifierwhich produces a stable DC voltage of 0.6 V from the alternating 13.56MHz RF signal. The middle waveforms (1710) are for reference voltages,while curves (1715) are for the three supply voltages VDD_S, VDD_A andVDD_D.

FIG. 18 shows the RF-wake-up block waveforms. The topmost waveform(1805) is for the amplitude modulated RF carrier (13.56 MHz) with a 17kHz message signal on top of it. This has to be demodulated andprocessed to retrieve the 17 kHz message signal from the carrier, asshown in the middle waveform (1810). The last waveform (1815) is theoutput of the digital block and is generated after mapping the durationof the message signal to a fixed value. This is used as a wake-up signalfor subsequent blocks.

FIG. 19 shows the waveforms of the data acquisition unit. The I2Cinterface block in this unit connects to the sensor through clock anddata signals, shown as (1905) and (1910) respectively. The datacollected is then transferred to the sensor interface block forprocessing. The processed data is illustrated as (1915). This is sent tothe backscatter switch to modulate the RF signal's amplitude at afrequency of 50 kHz. The enable signal is illustrated as (1920) andcontrols the data flow direction between the sensor and the ASIC chip.

FIG. 20 shows the backscatter modulation waveforms. The modulated signalappears across the RF coils, as shown in (2005) and is received by anexternal receiver. The modulating signal is shown in (2010). When thissignal is low, the rectifier consumes major part of the current as shownin (2015). Whereas, when this signal is high, the backscatter switchsinks almost all of the current as shown in (2020), and produces therequired modulation on the RF signal.

In some embodiments, the magnetic field gradient coils needs to producea field gradient of 30 mT/m (in each direction) in order to achieve aspatial resolution of 100 μm for the given sensor resolution of 3.1 μT.It is possible to reduce the volume of the gradient coils drasticallyand obtain the above field values by using coils that measure 30 cm×30cm×1 cm, as shown in FIGS. 21-22. FIG. 21 illustrates a top view of thecoils, in perspective, while FIG. 22 illustrates the bottom view. Thisresult was possible by increasing the current density for each coilsetup, and by using higher current values.

When the magnetic field gradients are simulated, it is observed that thex coils, in this example, need a current of 18 A, the y coils need 20 A,and the z coils need 13 A to generate a gradient of 30 mT/m in all threedimensions. The simulated gradient plots are shown in FIGS. 23-25. Thehigh currents could generate serious heating issues for the coils. Toavoid this problem time-multiplexing of the field measurement is carriedout, which turns the gradient coils ON and produces a stable gradientfor only 1 ms, the time needed by sensor to make a measurement. For theremainder of the time slot, which is approximately 80 ms in every timeframe of 100 ms, the corresponding gradient coils are OFF and theheating effect is negligible. The 20 ms time window for which the coilsare ON also takes into account the finite rise and fall time of thecurrent, due to self-inductance of the coils.

In some embodiments, the coils can be fabricated with litz wire. Litzwire is a specialized multistrand wire that reduces skin effects andproximity losses. The wire can be wound on a 3D printed plastic base. Avery good match can be observed between the simulated (2605,2705) andmeasured (2610,2710) gradient profiles for exemplary coils fabricated asprototypes, as shown in FIGS. 26 and 27 for the z and x coilsrespectively. The gradient profile for the y coils was similar to thatof the x coils. The magnetic sensors in the devices can detect threefield components in three orthogonal axes. The system can displayrelative alignment and locations of the first and second sensorsattached, for example, to a surgical nail for bone repair, and asurgical drill.

In some embodiments, the surgical device, e.g. the drill, can identifyits location without the use of ATOMS, for example with gyroscopes oroptical positioning, while the nail is located using ATOMS. In someembodiments, the surgical device does not transmit its data wirelessly,but it is instead wired to the computer or the display.

The examples set forth above are provided to those of ordinary skill inthe art as a complete disclosure and description of how to make and usethe embodiments of the disclosure, and are not intended to limit thescope of what the inventor/inventors regard as their disclosure.

Modifications of the above-described modes for carrying out the methodsand systems herein disclosed that are obvious to persons of skill in theart are intended to be within the scope of the following claims. Allpatents and publications mentioned in the specification are indicativeof the levels of skill of those skilled in the art to which thedisclosure pertains. All references cited in this disclosure areincorporated by reference to the same extent as if each reference hadbeen incorporated by reference in its entirety individually.

It is to be understood that the disclosure is not limited to particularmethods or systems, which can, of course, vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting. As used in this specification and the appended claims, thesingular forms “a,” “an,” and “the” include plural referents unless thecontent clearly dictates otherwise. The term “plurality” includes two ormore referents unless the content clearly dictates otherwise. Unlessdefined otherwise, all technical and scientific terms used herein havethe same meaning as commonly understood by one of ordinary skill in theart to which the disclosure pertains.

What is claimed is:
 1. A system comprising: a first sensor configured tobe inserted in a patient during a surgical procedure, the first sensorcomprising: a first magnetic sensor configured to detect a firstmagnetic field value, a first integrated circuit chip configured toprocess data from the first magnetic sensor, and a first radiofrequencycoil configured to transmit data processed by the first integratedcircuit chip based on the first magnetic field; a second sensor attachedto a surgical instrument, the second sensor comprising: a secondmagnetic sensor configured to detect a second magnetic field value, anda second integrated circuit chip configured to process data from thesecond magnetic sensor; and a plurality of coils configured to generatea magnetic field gradient within a volume in which the surgicalprocedure takes place, wherein the magnetic field gradient has a uniquefield value at each spatial location; wherein the plurality of coilscomprises at least: a first elliptical x coil and a second elliptical xcoil configured to accept currents flowing in opposite directions toeach other; a first elliptical y coil and a second elliptical y coilconfigured to accept currents flowing in opposite directions to eachother, the first and second elliptical y coils laying in a planeparallel to the first and second elliptical x coils, and rotated 90°relative to the first and second elliptical x coils; and a z coil. 2.The system of claim 1, wherein the first sensor further comprises abattery.
 3. The system of claim 1, wherein the first radiofrequency coilof the first sensor is further configured to wirelessly receive power.4. The system of claim 1, wherein the first magnetic sensor and thesecond magnetic sensor are Hall sensors configured to sense three fieldcomponents in three orthogonal axis, and the magnetic field gradient ismonotonic.
 5. The system of claim 1, wherein the first sensor isconfigured to be attached in a surgical nail to be inserted in a humanbone, the second sensor is configured to be attached to a surgicaldrill, and the surgical procedure comprises alignment of the surgicaldrill and of the surgical nail.
 6. The system of claim 1, wherein thefirst and second magnetic sensors have a magnetic field resolution of atleast 3.1 μT, an average power consumption of less than 10 μW, and afield measurement dynamic range of ±35 mT for each magnetic field axiscomponent.
 7. The system of claim 1, wherein the magnetic field gradientis 30 mT/m.
 8. The system of claim 1, wherein the first integratedcircuit chip is further configured to time multiplex power allocated tothe first magnetic sensor and to the first radiofrequency coil.
 9. Thesystem of claim 8, wherein multiplexing of the first integrated circuitchip comprises: a wake up signal to the first magnetic sensor, receivedby the first radiofrequency coil; a first time allocation to detect an xcomponent of the magnetic field gradient; a second time allocation todetect a y component of the magnetic field gradient; a third timeallocation to detect a z component of the magnetic field gradient; adata transmission through the first radiofrequency coil; and a sleepsignal to the first magnetic sensor.
 10. The system of claim 1, furthercomprising a second radiofrequency coil configured to transmit dataprocessed by the second integrated circuit chip based on the secondmagnetic field.
 11. A system comprising: a first sensor configured to beinserted in a patient during a surgical procedure, the first sensorcomprising: a magnetic sensor configured to detect a first magneticfield value, an integrated circuit chip configured to process data fromthe magnetic sensor, and a radiofrequency coil configured to transmitdata processed by the integrated circuit chip based on the firstmagnetic field; a surgical instrument; a second sensor configured tosense a location of the surgical instrument relative to the firstsensor; and a plurality of coils configured to generate a magnetic fieldgradient within a volume in which the surgical procedure takes place,wherein the magnetic field gradient has a unique field value at eachspatial location; wherein the plurality of coils comprises at least: afirst elliptical x coil and a second elliptical x coil configured toaccept currents flowing in opposite directions to each other; a firstelliptical y coil and a second elliptical y coil configured to acceptcurrents flowing in opposite directions to each other, the first andsecond elliptical y coils laying in a plane parallel to the first andsecond elliptical x coils, and rotated 90° relative to the first andsecond elliptical x coils; and a z coil.
 12. The system of claim 11,wherein the second sensor comprises a gyroscope attached to the surgicalinstrument, or an optical positioning device.
 13. A system comprising: afirst sensor configured to be inserted in a patient during a surgicalprocedure, the first sensor comprising: a first magnetic sensorconfigured to detect a first magnetic field value, a first integratedcircuit chip configured to process data from the first magnetic sensor,and a first radiofrequency coil configured to transmit data processed bythe first integrated circuit chip based on the first magnetic field; asecond sensor attached to a surgical instrument, the second sensorcomprising: a second magnetic sensor configured to detect a secondmagnetic field value, and a second integrated circuit chip configured toprocess data from the second magnetic sensor; and a plurality of coilsconfigured to generate a magnetic field gradient within a volume inwhich the surgical procedure takes place, wherein the magnetic fieldgradient has a unique field value at each spatial location; wherein thefirst integrated circuit chip is further configured to time multiplexpower allocated to the first magnetic sensor and to the firstradiofrequency coil; and wherein multiplexing of the first integratedcircuit chip comprises: a wake up signal to the first magnetic sensor,received by the first radiofrequency coil; a first time allocation todetect an x component of the magnetic field gradient; a second timeallocation to detect a y component of the magnetic field gradient; athird time allocation to detect a z component of the magnetic fieldgradient; a data transmission through the first radiofrequency coil; anda sleep signal to the first magnetic sensor.